Many sensors used to monitor physical conditions in process control systems or the like are inherently non-linear such that a correction operation must be carried out before the measured quantity can be displayed or used in calculations. By way of example, thermocouples are very widely used to measure temperature, but thermocouples are notoriously non-linear. Furthermore, different types of thermocouples display different non-linear characteristics. The classical approach to converting a voltage measurement obtained from a thermocouple to a temperature has involved empirically segmenting the characteristic curve of the particular type of thermocouple in order that a different formula may be applied for each segment in which a reading may be obtained. This approach is complex, time consuming, and is not sufficiently accurate for many precision applications since the characteristic curve is simulated by more or less coarse straight line segments.
A precision measurement of the voltage generated by a thermocouple may be obtained utilizing the well-known double-slope analog measuring technique. The amplified signal from the thermocouple is integrated for a predetermined time period from which a voltage representative of the voltage generated by the thermocouple is obtained. Then the double-slope measuring apparatus determines precisely what this voltage is by measuring the time necessary to discharge the holding component to zero when the discharge is carried out at a constant slope. The time period for carrying out the discharge can, of course, be reckoned by counting the number of pulses from a precision clock source during this integrate reference period. However, as previously suggested, the accumulated count obtained will not accurately reflect the measured temperature because of the inherent non-linearity of the thermocouple.
It has been proposed to linearize the clock pulses which are allowed to increment a counter during the integrate reference phase. That is, the clock pulses would be frequency varied in accordance with the slope of the various segments of the sensor's characteristic curve during the integrate reference period. It has further been proposed to carry out this function by passing a constant frequency clock signal through a synchronous binary rate multiplier, the fractional multiplication factor of which is controlled by a plurality of binary input signals methodically applied in accordance with the particular segments of the sensor's characteristic curve through which the measuring apparatus is passed. In effect, the characteristic curve is separated into straight line segments without the necessity for actually calculating according to different formulas for each segment. Thus, this approach also does not give sufficient accuracy for many applications.